MODULE SWAPS BETWEEN RELATED TRANSLOCATOR PROTEINS PIV(F1), PIV(IKE) AND PULD - IDENTIFICATION OF A SPECIFICITY DOMAIN

In Gram-negative bacteria, type II and type III secretion and filament ous phage assembly systems use related outer membrane proteins for sub strate-specific transport across the outer membrane. We show here that the specificity domain of the phage f1 outer membrane protein pIV is contained within the 149 N-terminal amino acid residues. When the pIV( f1) specificity domain is fused to the translocator domain of the rela ted pIV of phage IKe, the chimeric construct supports f1 but not IKe a ssembly. Functional coupling between the two domains in this chimeric construct is poor and is improved by a single amino acid change in the translocator domain of the pIV(IKe). In native pIV(IKe), two amino ac id changes within its specificity domain are both necessary and suffic ient to change the specificity from IKe to f1 assembly. Analysis of 39 chimeric constructs between pIV(f1) and the outer membrane protein Pu lD of the pullulanase secretion system failed to identify a comparable exchangeable specificity domain. These results indicate that the two domains may not function autonomously, and suggest that tertiary and q uarternary changes of the entire translocator component rather than of an autonomous functional domain are required for specific translocati on across the outer membrane. (C) 1997 Academic Press Limited.